The black water system on an RV does NOT work the same way as your home sewer system. Be sure to leave the black water valve CLOSED. Also, after hook up, put a few gallons of water down the toilet into the holding tank along with whatever chemical you choose to use. When the tank is full, open the valve and let it drain, then close it up, and replenish the chemical and those all important first few gallons of water. If you leave the valve open, you will end up with a rather unpleasant, solid pyramid of waste in the middle of the tank. At the end of your stay, drain the black water tank first, then the grey water so the grey water can flush your sewer lines.